1. Field
The present invention relates to circuits for use in digital signaling, for example.
2. Background
As is well-known, devices and/or peripherals may be coupled to a computer, such as a personal computer (PC), employing a signaling scheme compliant with the standard Universal Serial Bus (USB) specification, version 1.0, available from USB-IF, 2111 NE 25th. Ave. MS-JF2-51, Hillsboro Oreg. 97124 (hereinafter, "USB specification"). However, currently signals complying with the USB specification are limited to twelve (12) megabits per second and no provision has been made for increasing this bandwidth. The addition of a high speed channel may provide a number of advantages in that it might relieve a bandwidth oversubscription problem that is sometimes present with devices complying with the USB specification and, likewise, it might permit new classes of devices, which may utilize bandwidths greater than 12 megabits per second, to be coupled with devices employing a signaling scheme that complies with the USB specification.
It would, of course, be desirable to permit devices compliant with the existing or standard Universal Serial Bus specification to operate within higher bandwidth, high speed channel, while maintaining support for devices that comply with the current or standard USB specification. Existing USB specification compliant interconnection or intercoupling schemes utilize a source termination scheme. Present signaling that complies with the USB specification is rail-to-rail in voltage signal level, referring to a scheme in which the voltage signal levels of the voltage sources are realized or achieved at the output signal, and limited in bit rate so that the reflected wave of the voltage transition may traverse the cable or bus twice (e.g., in a forward and backward direction) and be absorbed back at the source. Through this mechanism, when the standard USB specification is complied with, a reflected wave that reaches a driver or circuit encounters an impedance which matches that of the cable, thereby permitting the reflected wave to be absorbed before the bus state (and hence the receiver's impedance) transitions. Unfortunately, because the rail-to-rail drivers or circuits do not provide a substantially constant impedance while switching, it is desirable for the reflected wave to reach the driver or circuit before the next bit transition. As a result, a standard USB specification compliant scheme is limited to 12 megabits per second. A need exists for a driver or signaling circuit that achieves the foregoing source termination while permitting a standard USB specification compliant scheme to support a higher bandwidth, high speed channel.